1. Field of the Invention
The present invention relates to a drop-on-demand (hereinafter abbreviated DOD) type ink jet head.
2. Description of the Prior Art
Among non-impact printers for which the market has been rapidly increasing, the simplest is an ink jet printer which is suitable for color printing, and there is also the so-called DOD type ink jet printer which ejects an ink droplet to form a dot and is the current standard among non-impact. Typical DOD type ink jet printers include a Kyser type which is disclosed in Japanese Patent Publication No. 12138/1978 and a thermal jet type which is disclosed in Japanese Patent Publication No. 59914/1986. However, these conventional printers have difficulties in that miniaturization of the device is difficult in the former and ink tends to be burnt due to the application of intense heat in the latter.
So as to simultaneously overcome the above-described defects, Japanese Laid-Open Patent Publication No. 252750/1988 proposes a shear mode type ink jet head. The structure and the operating principle will be described with reference to FIGS. 9(a) and 9(b). FIG. 9(a) is a cross-sectional view illustrating the structure when not being driven and in which barriers 95ab, 95bc and 95cd are adhered on an insulating board 91 made of glass or ceramic at regular intervals and in parallel with each other to form a number of narrow channels 92a, 92b and 92c which form an ink chamber and ink passages. One end of each of the channels 92a, 92b and 92c is designed so that ink can be supplied from a common ink reservoir. The other end is adhered to a nozzle plate which has small nozzle holes 93a, 93b and 93c formed therein. Furthermore, the barriers are flexibly adhered to a cover 98 made of glass or ceramic by intervening elastic members 21. Here, the barriers are made of a piezoelectric material such as lead zirconate titanate (PZT) and are polarized in one direction as shown by an arrow 22 or in a direction opposite the arrow 22. On the wall surfaces of the respective barriers are formed electrodes 94a2, 94b1, 94b2 and 94c1. Subsequently, if a sufficient magnitude of positive electric potential with respect to the electrode 94b1 is applied to the electrode 94a2 of FIG. 9(a), the barrier 95ab undergoes a shear mode deformation as shown in FIG. 9(b). If an identical operation is performed on the barrier 95bc, wherein the electrodes 94b1 and 94b2 usually have an identical electric potential, the cross-section of the channel 92b for the ink chamber and the ink passage is reduced from the initial state of FIG. 9(a) to the state of FIG. 9(b). Namely, if ink is charged in the channel, the pressure of the ink is instantaneously raised to eject an ink droplet from the nozzle hole 93b.
FIG. 8 is a perspective view of the ink jet head thus formed and is similar to the one disclosed in Japanese Laid-Open Patent Publication No. 252750/1988. This head comprises barriers 85 made of a piezoelectric material and which are adhered to an insulating board 81, narrow channels 82 constituting an ink chamber and an ink passage, a nozzle plate 80 which is adhered to the channels so as to close ends thereof, and a cover 88 which covers the entire portion having the channels therein. Nozzle holes 83 open through the nozzle plate and eject ink droplets as described in connection with FIGS. 9(a) and 9(b). Further, the ink is introduced from an ink-supplying inlet 86 and is supplied to the respective channels through a common ink reservoir 87.